A normal human ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the bones of the middle ear 103 that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode. Although the following discussion is specific to cochlear implants, some hearing impaired persons are better served when the stimulation electrode is implanted in other anatomical structures. Thus hearing implant systems include brainstem implants, middle brain implants, etc. each stimulating a specific auditory target in the hearing system.
FIG. 1 also shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external behind the ear (BTE) processor 111 in which various signal processing schemes can be implemented. The processed signal is then converted into a digital data format for transmission by external transmitter coil 107 into the implant stimulator 108. Besides receiving the processed audio information, the implant stimulator 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective electrical stimulation of the cochlea 104.
Bilateral cochlear implant systems provide cochlear implants to both the left and right ears. Normally two sequential implantation surgeries are performed. The first surgery is performed on one side, and the second surgery is then performed some time later, sometimes years after the first implantation surgery, and even different surgeons may perform the separate surgeries. During the first implantation surgery, no data is documented on the exact position (e.g. regarding implantation angle) that can be used for the second side.
During both surgeries, the fixation position of implant stimulator is very important. The length of the connecting cable from the BTE processor to the external transmitter coil very much depends on the position of the implant stimulator. And in a bilateral implant system with a single external processor, the sound received by the processor from both sides should be symmetrical, otherwise there could be undesired inter aural time differences.